1. Field of the Invention
This invention relates to a novel sulfonium salt and a chemically amplified positive resist composition containing the same and suitable for use in fine patterning.
2. Prior Art
As the LSI (large scale integration) technology tends toward higher integration and higher speed, further refinement of pattern rules is required. Deep-ultraviolet lithography is regarded promising as the next generation of fine patterning technology. The deep-UV lithography is capable of working on the order of 0.3 to 0.4 .mu.m. If a less light absorbing resist is used, it is possible to form a pattern having a side wall nearly perpendicular to the substrate. Great attention is now paid to the technique of utilizing a high illuminance KrF excimer laser as a deep-UV light source. In order to employ this technique on a mass production scale, a resist material having low light absorption and high sensitivity is desired.
From this point of view, a number of chemically amplified positive working resist materials using acid catalysts were recently developed as disclosed in EP 249139, U.S. Pat. Nos. 4,491,628 and 5,310,619. These materials have high sensitivity, resolution and dry etching resistance and are promising as resist materials especially suited for deep-UV lithography.
It is known that the function of chemically amplified positive resist materials is largely affected by photoacid generators used therein. Typical photoacid generators are onium salts as shown below. ##STR1##
The onium salts themselves are lipophilic. When blended as a resist component, they act to reduce the solubility of the resist material in alkaline aqueous solution and to prevent the resist film from thinning upon development.
However, in exposed areas of positive resist material, photoacid generators absorb actinic radiation to decompose into products which are also lipophilic. The decomposed products reduce the rate of dissolution of the exposed areas in alkaline aqueous solution, failing to provide a high ratio of the alkali dissolution rate of exposed areas to that of unexposed areas (which ratio is known as dissolution contrast).
This problem can be solved by incorporating a tert-butoxycarbonyl group or acid labile group into p-hydroxyphenylsulfonium salts as disclosed in JP-A 26550/1989, 35433/1989, and 12153/1990. Upon exposure to actinic radiation, the salts decompose to generate acids, which help form alkali soluble phenol derivatives, providing an enhanced dissolution contrast.
These tert-butoxycarbonyloxyphenylsulfonium salts, however, lack thermal stability and fail to satisfy the requirement of high resolution. Additionally, they generate strong acids such as metal halide anions and trifluoromethanesulfonic acid. Since the generated acid has high acidity, an acid labile group can be effectively decomposed with a small amount of acid. However, since the amount of acid generated is very small, line patterns would have a T-top profile, that is, patterns become thick at the top if the leave-to-stand or delay time from exposure to post-exposure baking (PEB) is extended. Patterns tend to be readily affected by contamination with air-borne basic substance.
Light exposure generates acid at the resist surface which reacts with air-borne basic substance and is thereby deactivated. As the delay time until PEB is extended, more acid is deactivated and accordingly, decomposition of acid labile groups is more unlikely to occur. As one solution, it is known from JP-A 232706/1993 and 249683/1993 to add a basic compound in a resist material for suppressing the influence of air-borne basic substance. According to our follow-up test, the basic compound used therein is poorly incorporated into the resist film due to volatilization, less compatible with resist components, and unevenly dispersible in a resist film. Thus the basic compound cannot achieve its advantages in a reproducible manner and causes a reduction of resolving power.
It is also reported in Proc. SPIE, 2195, 74-83 (1994) that strong acids such as trifluoromethanesulfonic acid resulting from photolysis reaction give rise to undesirable side reaction during PEB step involving decomposition of a tert-butoxycarbonyloxyphenyl group or acid labile group, forming a by-product having a hydroxyphenyl group tert-butylated at the o-position which causes a reduction of alkali solubility.